More details for SC-4 (shared system resources)

[shawndwells]

- control_key: SC-4
  standard_key: NIST-800-53
  covered_by: []
  implimentation_status: none
  narrative:
    - text: |
        '<The customer is responsible for preventing unauthorized and
        unintended information transfer via shared system resources. A
        successful control response will discuss how this is prevented.>'

maybe tell sVirt story?